It has been said that the fatigue strength of high strength bolts does not increase in proportion to the statical tensile strength. In order to establish the stable method for the improvement of fatigue strength of high strength bolts, the large scale experiments are done, referring to the published papers. The six factors including the pitch modification and the sequence of thread finishing are taken, and the design of experiment is based on the orthogonal array L32 (231). As the method of fatigue testing, the staircase method with small samples is used. The obtained conclusions are as follows : (1) In the range of the present experiments, the effects of nominal diameter and pitch of screw threads are not significant, and the effects of pitch modification, strength grade (D), sequence of thread finishing and mean stress in fatigue testing (G) as well as the interaction between D and G are significant. (2) The slight reducing (-0.3%) of the pitch of bolt threads comparing with that of nut threads and the thread rolling after heat treatment are effective to improve the fatigue strength of high strength bolts, and especially, the use of both actions has an excellent effect.